Aerosol cans generally have a structure in which the content is sprayed outside the can by using the internal pressure. As a material of the can, a steel sheet is frequently used such that the can endures the internal pressure. Moreover, the aerosol can includes a container constituted with three members including a can body part, a mountain cap, and a bottom cover. The material of the members is selected, or the shape thereof is designed, such that the members endure the internal pressure.
Among the members, the bottom cover is prepared in a manner in which a steel sheet is punched in a circular shape, and then the circular steel sheet is molded into a dome shape mainly by press forming. The bottom cover is attached to the can body part by seaming. The convexity of the dome shape of the bottom cover is installed to protrude toward the inside of the can such that the bottom cover is attached to the can body part, whereby the bottom cover plays a role of dispersing the internal pressure and maintaining strength of the can.
For the materials for the bottom cover of an aerosol can that is provided for the above use, 4 types of mechanical characteristics including pressure resistance, shape fixability, airtightness, and stretcher-strain resistance (hereinafter, stretcher-strain will be described as St-St) are required.
Among the mechanical characteristics, the pressure resistance of a steel sheet is specified mainly by yield point (YP). As a technique for improving the pressure resistance, a method (solid solution strengthening) of making a solid solution remain in steel, a method (work strengthening) of introducing dislocation into steel by temper rolling (hereinafter, abbreviated to TR in some cases), or the like is mainly used. In the process of the related art in which C and N are added in an appropriate amount to steel so as to secure a solid solution, and then general TR of a rolling ratio of about 1% is performed, YP is 400 to 450 MPa. On the other hand, in a so-called 2CR process (two times of cold rolling) in which TR is performed at a rolling ratio of 20 to 30% by using a lubricant, YP of the material can be reliably increased to 500 MPa or higher. However, the high YP is obtained by work strengthening, and moving dislocation cannot be newly introduced into the material. Accordingly, total elongation of the material is only several %.
In view of the shape fixability and airtightness, a steel sheet having excellent total elongation is preferable, and accordingly, it is difficult to make the shape fixability and airtightness compatible with a high degree of pressure resistance. However, so far, even if a relatively soft steel sheet of which the temper degree reaches up to T-5 level specified by JIS G 3303 is used as a bottom cover of an aerosol can, a high internal pressure which may cause a problem in the pressure resistance is rarely applied to the aerosol can. Therefore, improvement of the steel sheet has been rarely required. Moreover, even if St-St occurs slightly in a steel sheet, the St-St is regarded just as a problem of the exterior, so there has been no steel sheet specially designed as a bottom cover of an aerosol can. Further, there has been no steel sheet for bottom covers of aerosol cans that is devised such that the steel sheet is soft at the time of press forming or seaming and exhibits increased strength after being made into a can.